After their first introduction in the early 1990s, wireline broadband networks, which includes fiber, coaxial cable and twisted pair, have evolved substantially. Despite the inherent attenuation of copper which limits the capacity, transmission over this medium remains attractive as it is abundantly present throughout the world, due to legacy Plain Old Telephony Service (POTS) deployment. Hence, broadband over copper offers substantial deployment cost savings as compared to Fiber-To-The-Home (FTTH). Indeed, while FTTH has been technologically viable since 1988, Digital Subscriber Line (DSL) remains the predominant broadband access technology for the residential market. However, as the access network remains the bottleneck in the end-to-end connection and due to the continuing demand for ever higher data rates, copper is being replaced by fiber step-by-step. The cost per user of fiber deployment increases substantially when moving closer to the subscriber premises. This is why different operators have expressed enthusiasm with recent technologies, such as phantom mode and vectoring, which hold the promise of delivering more than 300 Mb/s. The success of vectoring and phantom mode transmission triggered interest in a next-generation broadband copper access beyond vectored vDsL2 to deliver 500 Mb/s to 1 Gb/s over relatively short loops.
Discrete Multi-Tone (DMT) modulation remains one of the prominent candidates for next generation access networks. Indeed, DMT is very flexible in the frequency domain and especially suited for spectral confinement, which is important when moving to higher bandwidths, where additional notching is required. Furthermore, as any other multi-carrier based modulation, the frequency selectivity of the channel can easily be addressed by very basic single tap equalizers.
However, Digital Subscriber Line (DSL) is not so robust against transient noise: DSL communications assume a quasi-stable channel and noise environment, and no standardized mechanism is foreseen to cope with fast channel variations.
In current DSL standards, the receiver determines the respective carrier bit loadings and gains based on Signal Noise plus Interference Ratio (SNIR) measurements carried out during channel initialization, and reports back these parameters for use by the transmitter. The receiver is also responsible for protecting against slow channel variations during showtime by means of bit swap, Seamless Rate Adaptation (SRA) or Save Our Showtime (SOS) procedures. Any noise fluctuation above the measured noise floor has to be masked with a noise margin, or with virtual noise. Typically, operators use noise margins from 5 to 15 dB on top of the line code limit, which represents a loss of about 10% to 40% in terms of achievable data rate. Noise bursts still above the noise margin are expected to be infrequent and short in duration (so-called impulsive noise), and hence can be corrected by means of Forward Error Correction (FEC) combined with data interleaving (at the expense of communication latency and data overhead), or by means of Automatic Repeat reQuest (ARQ).
With vectoring, the crosstalk interference levels at the receivers are considerably reduced. Therefore, substantial noise variations, previously masked by a high and stable crosstalk level, will become visible. Hence, operators may require even larger noise margins to cope with this increase in noise dynamics, thereby reducing the projected vectoring gain.
Still further, due to continued fiber deployment, which reduces the length of the copper loops, DSL band plans are extended to higher frequencies. These frequencies have higher crosstalk coupling, but also stronger pickup of non DSL noises such as Radio Frequency Ingress (RFI). Also here, we can expect an increase in noise dynamics.
Also, there is a constant push by operators for reducing the overall power consumption of the access plant. Power reduction is also a ‘fairness’ mechanism that could allow loop unbundling.
In short, current DSL systems are not able to provide end-users with optimal data rates at a right Quality of Service (QoS), and with optimal power per bit.